v3dv/descriptor: handle not having a sampler when combining texture and sampler id

v3dv/descriptor: handle not having a sampler when combining texture and sampler id

There are some texture operations (like mipmap query levels) that
doesn't require a sampler. In fact, you should ignore it. So we need
to take it into account when combining the
indexes. nir_tex_instr_src_index is returning a negative value to
identify that case, but as we are using a uint32_t to pack both values
(for convenience, easy to pack/unpack the hash table key), we just use
a uint value big enough to be a wrong sampler id.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>